1. FIELD OF THE INVENTION
This invention relates to a memory card with a built-in semiconductor memory element.
1. DESCRIPTION OF THE RELATED ART
Memory cards incorporating RAM (Random Access Memory), ROM (Read Only Memory), etc. are generally designed with the principal aim of providing low electric power consumption for various applications of portable equipment. RAMs as well as ROMs therefore basically use a CMOS logic type element. The CMOS element's electric power consumption is very low as compared with other NMOS elements. It is therefore essential to utilize the CMOS element for a RAM memory card particularly requiring a battery backup in order to provide the RAM memory card with a long service life having a minimal battery consumption during the backup.
The CMOS element is liable to incur a latch-up phenomenon from the viewpoint of its logical structure. The latch-up phenomenon involves a continuous overcurrent flow substantially the same as a short circuit current between the power source of the CMOS element and ground, which is due to various causes. This phenomenon leads ultimately to permanent thermal destruction. When this happens, the removal of the cause of the phenomenon and temporarily stopping the power to the CMOS element causes the latch-up phenomenon to disappear, whereby the CMOS or memory may be reused.
The latch-up phenomenon is likely to occur when the card is inserted or taken out. When the card is inserted, and the connectors of the card and of a host where the card is used are connected, an unexpected order in connection, i.e., first, a power source line is connected, then a ground line, and then an input and output signal line may take place, and a difference in voltage may also occur. That is, the electric potential of the input or the output signal line increases more than that of the power source line owing to the fact that the input and output signal line is connected earlier than the power source line. The foregoing is one of the causes for the latch-up phenomenon. Inversely, if the power source line is disconnected much earlier than the input and output signal line, the electric potential of the input and output line increases more than that of the power source. Short circuiting between the power source of the CMOS element and ground caused by the above conditions may result in damage to the element. When the damage is not severe, only the memory is destructed, but in case of serious damage, the memory emits smoke and burns out, ultimately leading to the burning of the equipment.
The conventional memory card provides no measures to protect against thermal destruction caused by the the above mentioned latch-up phenomenon, i.e., an overcurrent between the power source of the memory element, RAM or ROM, and ground. As stated above, since the latch-up phenomenon involves a continuous overcurrent flow substantially the same as a short circuit between the power source and ground, the greater the capacity of power fed to the memory card from the host, the greater the thermal destruction becomes, resulting in burning of the equipment.